Improved DDTeleporter

Made a minor change to DDTeleporter - added a flag so that we can
specify that entities must be teleported to the destination without
trying to shift them around based on destination orientation. This is
important for the trapdoor, since otherwise entities might get shoved
into walls. Also cleaned up some code in DDTeleporter. It's much more
readable now. These changes are in preparation for completing Trans
Trapdoor destination selection.